Re: [PATCH 1/2] KVM: x86: disable MPX if host did not enable MPX XSAVE features

[Yang Zhang]

On 2016/3/11 20:09, Paolo Bonzini wrote:

Sorry for the late reply.

> On 11/03/2016 03:37, Yang Zhang wrote:
> > > @@ -97,7 +104,7 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
> > >    	if (best && (best->eax & (F(XSAVES) | F(XSAVEC))))
> > >    		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> > >
> > > -	vcpu->arch.eager_fpu = use_eager_fpu() || guest_cpuid_has_mpx(vcpu);
> > > +	vcpu->arch.eager_fpu = use_eager_fpu();
> >
> > Hi Paolo,
> >
> > As i mentioned on another thread, force KVM to use eager fpu
> > unconditionally may introduce the performance regression. Though the
> > cost for eager fpu is very small especially in modern CPU, it still
> > cannot be ignored on old platform.
>
> This patch doesn't change anything in that respect.  It doesn't enable
> eager FPU in any case where it wasn't already enabled before the patch.

I mean why not keep the old way that only activate the eager_fpu while 
guest sees the MPX bit in CPUID, like:

vcpu->arch.eager_fpu = use_eager_fpu() && guest_cpuid_has_mpx(vcpu);

Besides, vmx_fpu_activate is called when do vcpu_reset. So it seems no 
need to call fpu_activate() here again.


--
best regards
yang